Mitigating the Handover Bottleneck in SWaP-constrained FSO terminals via NDOB-augmented control

Compact, lightweight Free Space Optical (FSO) terminals used in terrestrial applications are highly susceptible to severe aerodynamic gusts and structural vibrations. Consequently, their design requirements differ significantly from those of massive, stationary gimbal systems. In dual-stage optical assemblies, conventional coarse-stage controllers frequently fail to reject these high amplitude transients, causing residual tracking errors to propagate directly to the micro-stage. This dynamic coupling creates a “Handover Bottleneck,” rapidly driving the highly constrained Fast Steering Mirror (FSM) into piezoelectric saturation and ultimately severing the optical link. To resolve this and actively prevent actuator saturation, we propose a novel control architecture utilizing a Nonlinear Disturbance Observer (NDOB) embedded within a Feedback Linearization (FBL) framework. By estimating and algebraically canceling composite environmental loads at the macro-stage, the NDOB-augmented controller introduces an “Active Electronic Stiffness” that effectively shields the fine-stage actuators. Evaluated via a high-fidelity nonlinear digital twin subjected to a severe composite aerodynamic disturbance spectrum, the proposed architecture successfully limits the Root Mean Square Stroke Consumption (SCR rms ) to under 15%. While baseline PID and nominal FBL architectures experience catastrophic saturation and link loss, our framework preserves over 85% of the FSM’s available dynamic range and achieves sub-milliradian ( < 100 μ rad) steady-state pointing precision. Ultimately, this approach provides a mathematically grounded, computationally efficient methodology to demonstrably enhance link survivability in terrestrial 5G/6G optical backhaul networks.